Function decorator
- Python course
- Intermediate
- Lesson 37/116
- 5 min
Learn to write your own function decorator.
Wrapping functions with arguments
A decorator can wrap any function. The wrapper may have zero, one or more parameters. The general solution: use *args, **kwargs to make the wrapper flexible.
Example: def wrapper(*args, **kwargs):\n return fn(*args, **kwargs). The wrapper then forwards whatever parameters arrive to the target. A fixed-parameter wrapper works only with specific signatures; the flexible signature is a common choice.
The two-layer structure
A decorator is a function that takes another function. Inside it a new function (the wrapper) is defined and returned. So a decorator is a higher-order function: it takes a function and returns a function.
Call order: @name is seen → name(target) is called → the wrapper is returned → the user calls the wrapper → the wrapper calls the target. This two-layer design gives decorators flexibility; parameterized decorators can have three layers.
Practice Write a flexible-signature decorator
Define a decorator named double. Start with def double(fn):; inside, define def wrapper(*args, **kwargs):. The wrapper body should have result = fn(*args, **kwargs) and return result * 2. End with return wrapper. Decorate def add(a, b): with @double, body return a + b. Finally call print(add(3, 4)).
Starter code
# Write the double decorator and the add functionSample solution
# Write the double decorator and the add function
def double(fn):
def wrapper(*args, **kwargs):
result = fn(*args, **kwargs)
return result * 2
return wrapper
@double
def add(a, b):
return a + b
print(add(3, 4))Expected output
14
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Complete the wrapper signature
def wrapper(____, **kwargs):
return fn(*args, **kwargs)def wrapper(*args, **kwargs):
return fn(*args, **kwargs)Explanation: *args collects positional arguments.
2 What must a decorator return?
- A list
- A class instance
- A number
- A new function Correct answer
Explanation: A decorator returns a new function that replaces the wrapped one.
3 Match the decorator layers
| Term | Meaning |
|---|---|
def call(fn) |
Outer layer |
def wrapper(*args) |
Inner layer |
return fn(...) |
Target call |
return wrapper |
Return wrapper |